170 HYDROGRAPHICAL SURVEYING [chap. v. 



At each of the stations B and C, the angle to A' being 

 observed, the difference between those angles and the angles 

 to A, which were recorded formerly, gives the distance in feet 

 A' 6 and A' c, from the formula — 



No. feet subtended x 34 



Angle in seconds = ,, . ., - 



distance m miles 



in a direction at right angles to B A' and C A' respectively. 



Having found the points h and c on the ground from these 

 measurements, the point A readily follows. 



CALCULATING A POSITION FROM TWO ANGLES TO 

 THREE KNOWN OBJECTS. 



It may be sometimes required, in the course of a survey not 

 regularly triangulated, to calculate the distance of the observer 

 from an object, from the two angles he has observed between 

 three known " points," one of them being the object whose 

 distance is required. Or he may require the angle, at the 

 object observed, to liim, from the same data. 



This is, perhaps, best accomplished by using the one-circle 

 method, so called in contradistinction to the method of pro- 

 traction by three circles already explained under " Station 

 Pointer." 



The three Figm'es 47- 49 give the three possible positions of 

 the objects, viz. : When the observer is inside the triangle 



